Chlamydiae are prokaryotes. They exhibit morphologic and structural similarities to gram-negative bacteria including a trilaminar outer membrane, which contains lipopolysaccharide and several membrane proteins that are structurally and functionally analogous to proteins found in E coli. They are obligate intra-cellular parasites with a unique biphasic life cycle consisting of a metabolically inactive but infectious extracellular stage and a replicating but non-infectious intracellular stage. The replicative stage of the life-cycle takes place within a membrane-bound inclusion which sequesters the bacteria away from the cytoplasm of the infected host cell.
C. pneumoniae is a common human pathogen, originally described as the TWAR strain of Chlamydia psittaci but subsequently recognised to be a new species. C. pneumoniae is antigenically, genetically and morphologically distinct from other chlamydia species (C. trachomatis, C. pecorum and C. psittaci). It shows 10% or less DNA sequence homology with either of C. trachomatis or C. psittaci. 
C. pneumoniae is a common cause of community acquired pneumonia, only less frequent than Streptococcus pneumoniae and Mycoplasma pneumoniae (Grayston et al. (1995) Journal of Infectious Diseases 168:1231; Campos et al. (1995) Investigation of Ophthalmology and Visual Science 36:1477). It can also cause upper respiratory tract symptoms and disease, including bronchitis and sinusitis (Grayston et al. (1995) Journal of Infectious Diseases 168:1231; Grayston et al (1990) Journal of Infectious Diseases 161:618; Marrie (1993) Clinical Infectious Diseases. 18:501; Wang et al (1986) Chlamydial infections Cambridge University Press, Cambridge. p. 329. The great majority of the adult population (over 60%) has antibodies to C. pneumoniae (Wang et al (1986) Chlamydial infections. Cambridge University Press, Cambridge. p. 329), indicating past infection which was unrecognized or asymptomatic.
C. pneumoniae infection usually presents as an acute respiratory disease (i.e., cough, sore throat, hoarseness, and fever; abnormal chest sounds on auscultation). For most patients, the cough persists for 2 to 6 weeks, and recovery is slow. In approximately 10% of these cases, upper respiratory tract infection is followed by bronchitis or pneumonia. Furthermore, during a C. pneumoniae epidemic, subsequent co-infection with pneumococcus has been noted in about half of these pneumonia patients, particularly in the infirm and the elderly. As noted above, there is more and more evidence that C. pneumoniae infection is also linked to diseases other than respiratory infections.
The reservoir for the organism is presumably people. In contrast to C. psittaci infections, there is no known bird or animal reservoir. Transmission has not been clearly defined. It may result from direct contact with secretions, from fomites, or from airborne spread. There is a long incubation period, which may last for many months. Based on analysis of epidemics, C. pneumoniae appears to spread slowly through a population (case-to-case interval averaging 30 days) because infected persons are inefficient transmitters of the organism. Susceptibility to C. pneumoniae is universal. Reinfections occur during adulthood, following the primary infection as a child. C. pneumoniae appears to be an endemic disease throughout the world, noteworthy for superimposed intervals of increased incidence (epidemics) that persist for 2 to 3 years. C. trachomatis infection does not confer cross-immunity to C. pneumoniae. Infections are easily treated with oral antibiotics, tetracycline or erythromycin (2 g/d, for at least 10 to 14 d). A recently developed drug, azithromycin, is highly effective as a single-dose therapy against chlamydial infections.
In most instances, C. pneumoniae infection is often mild and without complications, and up to 90% of infections are subacute or unrecognized. Among children in industrialized countries, infections have been thought to be rare up to the age of 5 y, although a recent study (E Normann et al, Chlamydia pneumoniae in children with acute respiratory tract infections, Acta Paediatrica, 1998, Vol 87, Iss 1, pp 23-27) has reported that many children in this age group show PCR evidence of infection despite being seronegative, and estimates a prevalence of 17-19% in 2-4 y olds. In developing countries, the seroprevalence of C. pneumoniae antibodies among young children is elevated, and there are suspicions that C. pneumoniae may be an important cause of acute lower respiratory tract disease and mortality for infants and children in tropical regions of the world.
From seroprevalence studies and studies of local epidemics, the initial C. pneumoniae infection usually happens between the ages of 5 and 20 y. In the USA, for example, there are estimated to be 30,000 cases of childhood pneumonia each year caused by C. pneumoniae. Infections may cluster among groups of children or young adults (e.g., school pupils or military conscripts).
C. pneumoniae causes 10 to 25% of community-acquired lower respiratory tract infections (as reported from Sweden, Italy, Finland, and the USA). During an epidemic, C. pneumonia infection may account for 50 to 60% of the cases of pneumonia. During these periods, also, more episodes of mixed infections with S. pneumoniae have been reported.
Reinfection during adulthood is common; the clinical presentation tends to be milder. Based on population seroprevalence studies, there tends to be increased exposure with age, which is particularly evident among men. Some investigators have speculated that a persistent, asymptomatic C. pneumoniae infection state is common.
In adults of middle age or older, C. pneumoniae infection may progress to chronic bronchitis and sinusitis. A study in the USA revealed that the incidence of pneumonia caused by C. pneumoniae in persons younger than 60 years is 1 case per 1,000 persons per year; but in the elderly, the disease incidence rose three-fold. C. pneumoniae infection rarely leads to hospitalization, except in patients with an underlying illness.
Of considerable importance is the association of atherosclerosis and C. pneumoniae infection. There are several epidemiological studies showing a correlation of previous infections with C. pneumoniae and heart attacks, coronary artery and carotid artery disease (Saikku et al. (1988) Lancet;ii:983; Thom et al. (1992) JAMA 268:68; Linnanmaki et al. (1993), Circulation 87:1030; Saikku et al. (1992) Annals Internal Medicine 116:273; Melnick et al (1993) American Journal of Medicine 95:499). Moreover, the organisms has been detected in atheromas and fatty streaks of the coronary, carotid, peripheral arteries and aorta (Shor et al. (1992) South African. Medical Journal 82:158; Kuo et al. (1993) Journal of Infectious Diseases 167:841; Kuo et al. (1993) Arteriosclerosis and Thrombosis 13:1500; Campbell et al (1995) Journal of Infectious Diseases 172:585; Chiu et al. Circulation, 1997 (In Press)). Viable C. pneumoniae has been recovered from the coronary and carotid artery (Ramirez et al (1996) Annals of Internal Medicine 125:979; Jackson et al. Abst. K121, p272, 36th ICAAC, 15-18 September 1996, New Orleans). Furthermore, it has been shown that C. pneumoniae can induce changes of atherosclerosis in a rabbit model (Fong et al (1997) Journal of Clinical Microbiolology 35:48). Taken together, these results indicate that it is highly probable that C. pneumoniae can cause atherosclerosis in humans, though the epidemiological importance of chlamydial atherosclerosis remains to be demonstrated.
A number of recent studies have also indicated an association between C. pneumoniae infection and asthma. Infection has been linked to wheezing, asthmatic bronchitis, adult-onset asthma and acute exacerbations of asthma in adults, and small-scale studies have shown that prolonged antibiotic treatment was effective at greatly reducing the severity of the disease in some individuals (Hahn D L, et al. Evidence for Chlamydia pneumoniae infection in steroid-dependent asthma. Ann Allergy Asthma Immunol. 1998 January; 80(1): 45-49; Hahn D L, et al. Association of Chlamydia pneumoniae IgA antibodies with recently symptomatic asthma. Epidemiol Infect. 1996 December; 117(3): 513-517; Bjornsson E, et al. Serology of chlamydia in relation to asthma and bronchial hyperresponsiveness. Scand J Infect Dis. 1996; 28(1): 63-69; Hahn D L. Treatment of Chlamydia pneumoniae infection in adult asthma: a before-after trial. J Fam Pract. 1995 October; 41(4): 345-351; Allegra L, et al. Acute exacerbations of asthma in adults: role of Chlamydia pneumoniae infection. Eur Respir J. 1994 December; 7(12): 2165-2168; Hahn D L, et al. Association of Chlamydia pneumoniae (strain TWAR) infection with wheezing, asthmatic bronchitis, and adult-onset asthma. JAMA. 1991 Jul. 10; 266(2): 225-230).
In light of these results a protective vaccine against C. pneumoniae infection would be of considerable importance. There is not yet an effective vaccine for any human chlamydial infection. It is conceivable that an effective vaccine can be developed using physically or chemically inactivated Chlamydiae. However, such a vaccine does not have a high margin of safety. In general, safer vaccines are made by genetically manipulating the organism by attenuation or by recombinant means. Accordingly, a major obstacle in creating an effective and safe vaccine against human chlamydial infection has been the paucity of genetic information regarding Chlamydia, specifically C. pneumoniae. 
Studies with C. trachomatis and C. psittaci indicate that safe and effective vaccine against Chlamydia is an attainable goal. For example, mice which have recovered from a lung infection with C. trachomatis are protected from infertility induced by a subsequent vaginal challenge (Pal et al. (1996) Infection and Immunity.64:5341). Similarly, sheep immunized with inactivated C. psittaci were protected from subsequent chlamydial-induced abortions and stillbirths (Jones et al. (1995) Vaccine 13:715). Protection from chlamydial infections has been associated with Th1 immune responses, particularly the induction of INFg—producing CD4+ T-cells (Igietsemes et al. (1993) Immunology 5:317). The adoptive transfer of CD4+ cell lines or clones to nude or SCID mice conferred protection from challenge or cleared chronic disease (Igietseme et al (1993) Regional Immunology 5:317; Magee et al (1993) Regional Immunology 5: 305), and in vivo depletion of CD4+ T cells exacerbated disease post-challenge (Landers et al (1991) Infection & Immunity 59:3774; Magee et al (1995) Infection & Immunity 63:516). However, the presence of sufficiently high titres of neutralising antibody at mucosal surfaces can also exert a protective effect (Cotter et al. (1995) Infection and Immunity 63:4704).
Antigenic variation within the species C. pneumoniae is not well documented due to insufficient genetic information, though variation is expected to exist based on C. trachomatis. Serovars of C. trachomatis are defined on the basis of antigenic variation in the major outer membrane protein (MOMP), but published C. pneumoniae MOMP gene sequences show no variation between several diverse isolates of the organism (Campbell et al. Infection and Immunity (1990) 58:93; McCafferty et al Infection and Immunity (1995) 63:2387-9; Gaydos et al. Infection and Immunity.(1992) 60(12):5319-5323). The gene encoding a 76 kDa antigen has been cloned from a single strain of C. pneumoniae and the sequence published (Perez Melgosa et al. Infection and Immunity.(1994) 62:880). An operon encoding the 9 kDa and 60 kDa cysteine-rich outer membrane protein genes has been described (Watson et al., Nucleic Acids Res (1990) 18:5299; Watson et al., Microbiology (1995) 141:2489). Many antigens recognized by immune sera to C. pneumoniae are conserved across all chlamydiae, but 98 kDa, 76 kDa and several other proteins may be C. pneumoniae-specific (Perez Melgosa et al. Infection and Immunity.(1994) 62:880; Melgosa et al., FEMS Microbiol Lett (1993) 112 :199; Campbell et al., J Clin Microbiol (1990) 28 :1261; Iijima et al., J Clin Microbiol (1994) 32:583). An assessment of the number and relative frequency of any C. pneumoniae serotypes, and the defining antigens, is not yet possible. The entire genome sequence of C. pneumoniae strain CWL-029 is now known and as further sequences become available a better understanding of antigenic variation may be gained.
Many antigens recognised by immune sera to C. pneumoniae are conserved across all chlamydiae, but 98 kDa, 76 kDa and 54 kDa proteins appear to be C. pneumoniae-specific (Campos et al. (1995) Investigation of Ophthalmology and Visual Science 36:1477; Marrie (1993) Clinical Infectious Diseases. 18:501; Wiedmann-Al-Ahmad M, et al. Reactions of polyclonal and neutralizing anti-p54 monoclonal antibodies with an isolated, species-specific 54-kilodalton protein of Chlamydia pneumoniae. Clin Diagn Lab Immunol. 1997 November; 4(6): 700-704).
Immunoblotting of isolates with sera from patients does show variation of blotting patterns between isolates, indicating that serotypes C. pneumoniae may exist (Grayston et al. (1995) Journal of Infectious Diseases 168:1231; Ramirez et al (1996) Annals of Internal Medicine 125:979). However, the results are potentially confounded by the infection status of the patients, since immunoblot profiles of a patient's sera change with time post-infection. An assessment of the number and relative frequency of any serotypes, and the defining antigens, is not yet possible.
Accordingly, a need exists for identifying and isolating polynucleotide sequences of C. pneumoniae for use in preventing and treating Chlamydia infection.